The invention relates to needling fiber structures, in particular to make preforms for constituting reinforcing structures in composite material parts, e.g. such as preforms for brake disks of thermostructural composite material.
To make such needled structures, it is well known to stack fiber plies on a platen and to needle the plies as they are being stacked by means of needles which are driven with reciprocating motion in a direction that extends transversely relative to the plies (or Z direction).
The needles take fibers from the plies and transfer them in the Z direction. The Z fibers confer cohesion and resistance to delamination (ply separation) to the needled structure. It is thus possible to ensure that composite parts incorporating such structures as fiber reinforcement have mechanical strength enabling them to withstand shear forces, as is necessary for brake disks when applying braking torque.
To confer desired needling characteristics throughout the thickness of the needled fiber structure, it is known to control the distance between the platen and one end of the needle stroke while the stack of plies is being built up. More particularly, document U.S. Pat. No. 4,790,052 proposes increasing this distance each time a new ply is stacked by causing the platen to move down by a step of size equal to the thickness of the needled ply, the purpose being to cause needling density to be uniform throughout the entire thickness of the fiber structure.
Document EP 0 736 115 proposes taking account of variation in the behavior of the fiber structure while it is being built up so that the size of the down steps imparted to the platen varies in compliance with a predetermined decreasing relationship. The purpose is to confer constant thickness to the various layers constituted by the needled-together plies.
Document EP 0 695 823 proposes transferring fibers in the Z direction by controlling needle penetration depth during the needling process. To this end, a magnitude representative of the position of the free surface of the fiber structure being needled is generated by using sensors which measure the position of the free surface outside the needling zone.
Compared with a process in which the size of the down step is predetermined, real time measurement of the position of the surface can make it possible to take account of any drift that may occur relative to a model, e.g. due to variations in the thicknesses of individual plies. Nevertheless, in Document EP 0 695 823, the measurement is not taken exactly in register with the needling. In addition, other kinds of drift are possible relative to the preestablished conditions, and these are not taken into account, for example needle wear.
An object of the invention is to provide a needling method that makes it possible to take account of the real effectiveness of the needles throughout the needling process, so as to be able to monitor or control the process in real time.
This object is achieved by a method of making a needled fiber structure of the type comprising stacking fiber plies on a platen, needling the plies together as the stack is built up by means of needles that are driven with reciprocating motion in a direction that extends transversely relative to the plies, and varying the distance between the platen and an end-of-stroke position of the needles while building up the stack so as to obtain a desired distribution of needling characteristics through the thickness of the fiber structure, in which method the instantaneous force (f) exerted during needle penetration is measured and a magnitude representing needling force (F) or penetration energy (E) is evaluated on the basis of the instantaneous force, and the evaluated magnitude (F; E) is verified for compliance with at least one predetermined condition.
The penetration energy (E) of the needles can be evaluated by integrating the measured instantaneous force (f), e.g. over a duration from entry of the needles into the fiber structure and arrival of the needles at the bottom of their stroke.
The evaluated magnitude can also be the maximum value (F) of the instantaneous needling force (f) as measured during penetration of the needles in the fiber structure.
Depending on the distribution desired for needling characteristics in the thickness of the fiber structure, it is verified that the magnitude representative of the needling force (F) or of the penetration energy (E) remains substantially constant, or complies substantially with a preestablished variation relationship.
In an aspect of the invention, the measured needling force (F) or penetration energy (E) provides means for monitoring proper operation of the needling, and needling is controlled in application of a predefined process, e.g. a platen down step of constant size, or a particular variation in the size of the down step as in document EP 0 736 115.
In another aspect of the invention, variation in the distance between the platen and an end-of-stroke position of the needles is controlled as a function of the evaluated value for the needling force (F) or the penetration energy (E).
In particular, when the distance between the platen and an end-of-stroke position of the needles is caused to vary in predetermined manner during the needling process, and when the evaluated magnitude (E) or (F) does not satisfy a predetermined condition, an additional modification of said distance is superposed on said variation, where appropriate.
In this aspect, variation in distance is servo-controlled so as to maintain the needling force or the penetration energy of the needles at a predetermined value or so as to comply with a predetermined variation relationship, depending on the distribution desired for the needling characteristics through the thickness of the fiber structure, and in particular the characteristic of Z fiber density.
In both aspects of the invention, measuring the force exerted or the energy expended during penetration of the needles makes it possible to take account of the real effectiveness of the needles and to integrate any variation, e.g. the individual thickness of an irregular ply or premature wear of the needles.
The instantaneous penetration force (f) is advantageously measured on the platen.
The invention also provides needling apparatus enabling the above methods to be implemented.
This object is achieved by means of apparatus comprising a platen on which fiber plies can be stacked, a plurality of needles carried by a support above the platen, drive means for driving the needle support so as to impart reciprocating motion to the needles in a direction that extends transversely relative to the platen, and means for varying the distance between the platen and an end-of-stroke position of the needles, which apparatus includes at least one force sensor suitable for delivering a signal representative of the instantaneous force exerted during penetration of the needles into the fiber plies stacked on the platen.